Emergency fire conditions ease in NSW

Emergency fire conditions have eased in NSW after a day where sweltering temperatures and strong winds fanned blazes that threatened properties.

Hundreds of firefighters battled dozens of fires across the state on Sunday, with fires near the Hawkesbury River and in the Southern Highlands threatening homes.

Emergency warnings were issued for residents of Lower Portland, on the Hawkesbury River, and Alpine, south of Sydney, as temperatures soared into the thirties in the afternoon.

About fifty properties came under threat at Lower Portland shortly after 4.30pm (AEDT), with concerned residents warned it was too late to leave the area and told they must seek shelter if the fire approached their home.

More than 100 firefighters worked to quell the blaze, which reportedly crossed the river at one point, while six aircraft water bombed the fire zone.

It was eventually brought under control around 7.30pm as cooler conditions had in impact and residents were allowed to return to their homes, with no damage to property reported.

Firefighters are expected to work through the night, with a watch and act warning now in place.

At Alpine, a small number of isolated rural properties between the Hume Highway and the Old Hume Highway came under threat about 5.30pm.

The fire was brought under control after about two hours and downgraded to a watch and act warning.

It was a tough day for firefighters as temperatures soared to 34C and strong winds fanned the flames of 66 fires burning across the state.

A cool change that swept across the greater Sydney region from 4.30pm brought some relief, with temperatures plummeting from 34C to 24C in half an hour.

However, strong winds of up to 80km/h in some areas continued to hamper firefighters.

RFS spokesman Matt Sun said firefighters had worked in tough conditions to quickly bring the most dangerous fires under control.

"Some good work was done by our firefighters today," he told AAP.

But despite a forecast of more temperate weather conditions - Monday is expected to see highs of 20C - firefighters still have a lot of work ahead of them.

Thirty-two fires were still burning out of control on Sunday evening and watch and act warnings were in place for three fires, at Pallamallawa, Goulburn and Walcha.

Meanwhile, the Ambulance Service of NSW responded to more than 170 calls for people with breathing difficulties over the weekend after smoke blanketed parts of the state.

A health alert for poor air quality was issued by NSW Health, with paramedics receiving dozens of call-outs for breathing difficulties and asthma-related problems.